Formation Of Hydrogen Bubbles Research Articles

Non-destructive electrochemical graphene transfer from reusable thin-film catalysts

We demonstrate an electrochemical method – which we term oxidative decoupling transfer (ODT) – for transferring chemical vapor deposited graphene from physically deposited copper catalyst layers. This copper oxidation-based transfer technique is generally applicable to copper surfaces, and is particularly suitable where the copper is adhered to a substrate such as oxidized silicon. Graphene devices produced via this technique demonstrate 30% higher mobility than similar devices produced by standard catalyst etching techniques. The transferred graphene films cover more than 94% of target substrates – up to 100mm diameter films are demonstrated here – and exhibit a low Raman D:G peak ratio and a homogenous and continuous distribution of sheet conductance mapped by THz time-domain spectroscopy. By applying a fixed potential of −0.4V vs. an Ag/AgCl reference electrode – significantly below the threshold for hydrogen production by electrolysis of water – we avoid the formation of hydrogen bubbles at the graphene–copper interface, preventing delamination of thin sputtered catalyst layers from their supporting substrates. We demonstrate the reuse of the same growth substrate for five growth and transfer cycles and prove that this number is limited by the evaporation of Cu during growth of graphene. This technique therefore enables the repeated use of the highest crystallinity and purity substrates without undue increase in cost.

Nucleation and growth of hydrogen bubbles on dislocations in tungsten under high flux low energy plasma exposure

A new mechanism for the nucleation and growth of hydrogen (H) bubbles on dislocations under plasma exposure of tungsten was recently proposed on the basis of direct ab initio calculations. Density functional theory calculations demonstrated that H atoms are strongly bound to a screw dislocation core and exhibit fast one-dimensional migration along its line. Once the number of hydrogen atoms trapped on a dislocation segment exceeds eight, the emission of a jog occurs thereby converting a pure HN cluster into a HN+1-jog configuration. On the basis of these results a kinetic model was formulated to evaluate the conditions (i.e., range of temperature and flux exposure) for the transformation of pure H clusters into supercritical hydrogen–vacancy clusters attached to the dislocation line. In this work, a parametric study employing the kinetic nucleation model was performed to derive the hydrogen bubble formation energy function that offers the best agreement with available experimental results. The obtained results allow one to rationalize the depth and temperature dependence of the experimentally observed hydrogen deposition after high flux low energy plasma exposure for ITER relevant conditions.

Spatially dependent cluster dynamics model of He plasma surface interaction in tungsten for fusion relevant conditions

In fusion reactors, plasma facing components (PFC) and, in particular, the divertor will be irradiated with high fluxes of low-energy (∼100 eV) helium and hydrogen ions. Tungsten is one of the leading candidate divertor materials for ITER and DEMO fusion reactors. However, the behaviour of tungsten under high dose, coupled helium/hydrogen exposure remains to be fully understood. The PFC response and performance changes are intimately related to microstructural changes, such as the formation of point defect clusters, helium and hydrogen bubbles or dislocation loops. Computational materials' modelling results are described here that investigate the mechanisms controlling microstructural evolution in tungsten. The aim of this study is to understand and predict sub-surface helium bubble growth under high flux helium ion implantation (∼1022 m−2 s−1) at high temperatures (>1000 K). We report results from a spatially dependent cluster dynamics model based on reaction–diffusion rate theory to describe the evolution of the microstructure under these conditions. The key input parameters to the model (diffusion coefficients, migration and binding energies, initial defect production) are determined from a combination of atomistic modelling and available experimental data. The results are in good agreement with results of an analytical model that is presented in a separate paper. In particular, it is found that the sub-surface evolution with respect to bubble size and concentration of the helium bubbles strongly depends on the flux and temperature.

Hydrogen bubble formation and evolution in tungsten under different hydrogen irradiation conditions

In order to see how hydrogen is behaving in tungsten and to understand the way bubbles form and grow up, specimens were irradiated by hydrogen ions from room temperature to 800°C to fluence of 2.25×1021m−2. Experimental results show that higher temperature helped bubble acquire higher internal pressure, causing interstitial loop punching to happen. In this process bubbles’ size grew and dislocation loops were formed but dislocation loops migrated away at and above 350°C. And bubble number density reached peak value at 600°C but then dropped dramatically at 800°C. Because continuously increasing temperature would cause small bubbles dissolution or leaking out. Besides, high temperature also prevented tiny bubbles growing to be visible under TEM observation by their reaching equilibrium pressure before reaching threshold pressure for loop punching. In the other set of experiments, specimens were irradiated by low hydrogen fluence of 1×1020m−2 at 600°C, in which case few hydrogen bubbles appeared. With further increasing irradiation fluence, bubble number density quickly increased. Small bubbles tended to coalesce to become larger visible bubbles. And they continued to grow through loop punching until their internal pressure cannot support their size expansion any more.

A novel anaerobic electrochemical membrane bioreactor (AnEMBR) with conductive hollow-fiber membrane for treatment of low-organic strength solutions.

A new anaerobic treatment system that combined a microbial electrolysis cell (MEC) with membrane filtration using electrically conductive, porous, nickel-based hollow-fiber membranes (Ni-HFMs) was developed to treat low organic strength solution and recover energy in the form of biogas. This new system is called an anaerobic electrochemical membrane bioreactor (AnEMBR). The Ni-HFM served the dual function as the cathode for hydrogen evolution reaction (HER) and the membrane for filtration of the effluent. The AnEMBR system was operated for 70 days with synthetic acetate solution having a chemical oxygen demand (COD) of 320 mg/L. Removal of COD was >95% at all applied voltages tested. Up to 71% of the substrate energy was recovered at an applied voltage of 0.7 V as methane rich biogas (83% CH4; <1% H2) due to biological conversion of the hydrogen evolved at the cathode to methane. A combination of factors (hydrogen bubble formation, low cathode potential and localized high pH at the cathode surface) contributed to reduced membrane fouling in the AnEMBR compared to the control reactor (open circuit voltage). The net energy required to operate the AnEMBR system at an applied voltage of 0.7 V was significantly less (0.27 kWh/m3) than that typically needed for wastewater treatment using aerobic membrane bioreactors (1-2 kWh/m3).

Critical concentration for hydrogen bubble formation in metals

Employing a thermodynamic model with previously calculated first-principle energetics as inputs, we determined the hydrogen (H) concentration at the interstitial and monovacancy as well as its dependence on temperature and pressure in tungsten and molybdenum. Based on this, we predicted the critical H concentration for H bubble formation at different temperatures. The critical concentration, defined as the value when the concentration of H at a certain mH-vacancy complex first became equal to that of H at the interstitial, was 24 ppm/7.3 GPa and 410 ppm/4.7 GPa at 600 K in tungsten and molybdenum in the case of a monovacancy. Beyond the critical H concentration, numerous H atoms accumulated in the monovacancy, leading to the formation and rapid growth of H-vacancy complexes, which was considered the preliminary stage of H bubble formation. We expect that the proposed approach will be generally used to determine the critical H concentration for H bubble formation in metals.

'Bubble-free' electrochemical delamination of CVD graphene films.

The production of large amounts of hydrogen bubbles, typical of electrochemical delamination methods based on the electrolysis of water, results in mechanical damage to graphene during the delamination, transfer, and drying steps. Here a novel 'bubble-free' delamination method is introduced which exploits the electrochemical dissolution of native copper oxide at a potential lower than that required for the formation of hydrogen bubbles, enabling the production of defect-free graphene stack.

Temporary Wafer Bonding by Polyelectrolyte Interlayers

Polyelectrolyte multilayers of just a few nanometers thickness were used for direct bonding of silicon, thermally oxidized silicon, and borosilicate wafers, respectively, and the bonded wafer pairs were characterized during and after annealing at temperatures up to 350 °C. Determination of surface energy as a measure of bond strength was performed in situ during annealing. The bond strengths show an interesting temperature dependence which can be utilized for temporary bonding of silicon wafers. Infrared transmission photographs as well as FTIR and XPS measurements suggest that the formation of hydrogen bubbles, generated by the reaction between water and silicon, contributes to the debonding at elevated temperatures.

Spatially dependent cluster dynamics modeling of microstructure evolution in low energy helium irradiated tungsten

In fusion reactors, plasma facing components (PFC) and in particular the divertor will be irradiated with high fluxes of low energy (∼100 eV) helium and hydrogen ions. Tungsten is one of the leading candidate divertor materials for ITER and DEMO fusion reactors. However, the behavior of tungsten under high dose, coupled helium/hydrogen exposure remains to be fully understood. The PFC response and performance changes are intimately related to microstructural changes, such as the formation of point defect clusters, helium and hydrogen bubbles or dislocation loops. Computational materials modeling has been used to investigate the mechanisms controlling microstructural evolution in tungsten following high dose, high temperature helium exposure. The aim of this study is to understand and predict helium implantation, primary defect production and defect diffusion, helium-defect clustering and interactions below a tungsten surface exposed to low energy helium irradiation. The important defects include interstitial clusters, vacancy clusters, helium interstitials and helium-vacancy clusters. We report results from a one-dimensional, spatially dependent cluster dynamics model based on the continuum reaction–diffusion rate theory to describe the evolution in space and time of all these defects. The key parameter inputs to the model (diffusion coefficients, migration and binding energies, initial defect production) are determined from a combination of atomistic materials modeling and available experimental data.

Anaerobic Electrochemical Membrane Bioreactor (AnEMBR) for Low-Strength Wastewater Treatment with Resource Recovery

A new hybrid system that combines a microbial electrolysis cell and membrane bioreactor using electrically conductive, porous, hollow-fiber membrane was developed to treat low organic strength solutions (300 mg COD/L) and recover energy in the form of biogas. This new system is called an anaerobic electrochemical membrane bioreactor (AnEMBR). The nickel-based hollow-fiber serves the dual function of cathode electrode for hydrogen evolution and membrane to filter the treated effluent. The system was operated for 70 days and recovered up to 71% of the substrate energy as methane rich biogas due to biological conversion of the hydrogen evolved at the cathode surface to methane by hydrogenotrophic methanogens. At an applied voltage of 0.5 V the energy density of the produced biogas exceeded the electrical energy required to operate the system by 53%. Findings also suggest that hydrogen bubble formation, cathode potential and localized alkaline pH at the cathode membrane surface may help mitigate membrane fouling.

Instantly Investigating the Adsorption of Polymeric Corrosion Inhibitors on Magnesium Alloys by Surface Analysis under Ambient Conditions

Surface engineering of magnesium alloys requires adequate strategies, processes and materials permitting corrosion protection. Liquid formulations containing corrosion inhibitors often are to be optimized according to the demands of the respective substrate and following the service conditions during its application. As an interdisciplinary approach, a combination of several techniques for instantly monitoring or elaborately analyzing the surface state of magnesium was accomplished in order to characterize the performance of new adsorbing sustainable amphiphilic polymers which recently were developed to facilitate a multi-metal corrosion protection approach. The application of established techniques like Contact Angle measurements and X-ray Photoelectron Spectroscopy investigations was supplemented by introducing related and yet faster online-capable and larger-scale techniques like Aerosol Wetting Test and Optically Stimulated Electron Emission. Moreover, an inexpensive setup was configured for scaling the inset and the extent of degradation processes which occur at local electrochemical circuits and lead to hydrogen bubble formation. Using these analytical tools, changes of the surface state of emeried AM50 samples were investigated. Even in contact with water, being a moderate corrosive medium, the online techniques facilitated detecting surface degradation of the unprotected magnesium alloy within some seconds. In contrast, following contact with a 1 weight% formulation of a polymeric corrosion inhibitor, surface monitoring indicated a delay of the onset of degradation processes by approximately two orders of magnitude in time. Mainly based on the spectroscopic investigations, the corrosion inhibiting effects of the investigated polymer are attributed to the adsorption of a primary polymer layer with a thickness of a few nanometers which occurs within some seconds. Immersion of magnesium for several hours brings up a protective film with around ten nanometers thickness.

Nanostructure of Aluminum Oxide Inclusion and Formation of Hydrogen Bubbles in Molten Aluminum

Hydrogen in molten aluminum is one of the major factors that lead to pore formation in the solidification process of aluminum castings. Previous research showed that aluminum oxide inclusion had a close correlation with the hydrogen content in molten aluminum. Though some researchers thought there must have been a relationship between surface morphology of the inclusion and hydrogen concentration in molten aluminum, very few documents have reported on the surface property of aluminum oxide inclusion. In the present work, AFM (Atomic Force Microscope) was first used to investigate surface morphology of aluminum oxide inclusion in molten aluminum. It was found that there were a large number of nanoscale micropores on the surface of aluminum oxide inclusion. The interior of the micropores was approximated as a tapered shape. These micropores were thought to be helpful to heterogeneous nucleation for hydrogen atoms aggregation because they provided necessary concentration fluctuation and energy undulation for the growth of hydrogen bubbles. Based on the nanostructure observed on the surface of aluminum oxide inclusion, a theoretical model was developed to describe the hydrogen pore formation in aluminum casting under the condition of heterogeneous nucleation.

Modelling and Measurement of Bubble Formation and Growth in Electroflotation Processes

Abstract Electroflotation is used in the water treatment industry for the recovery of suspended particles. In this study the bubble formation and release of hydrogen bubbles generated electrolytically from a platinum cathode was investigated. Previously, it was found that both the growth rate and detachment diameter increased with increasing wire diameter. Conversely, current density had little effect on the released bubble size. It was also found that the detached bubbles rapidly increased in volume as they rose through the liquid as a result of decreasing hydrostatic pressure and high levels of dissolved hydrogen gas in the surrounding liquid. The experimental system was computationally modelled using a Lagrangian-Eulerian Discrete Particle approach. It was revealed that desorption of gaseous solutes from the electrolyte solution, other than hydrogen, may have a significant impact on the diameter variation of the formed bubbles. The simulation confirmed that liquid circulation, either forced or induced by the rising bubble plume, influences both the hydrogen supersaturation (concentration) in the neighbourhood of the electrode and the size of the resulting bubbles.

Field-Scale Transport and Transformation of Carboxymethylcellulose-Stabilized Nano Zero-Valent Iron

The fate of nano zerovalent iron (nZVI) during subsurface injection was examined using carboxymethylcellulose (CMC) stabilized nZVI in a very large three-dimensional physical model aquifer with detailed monitoring using multiple, complementary detection methods. A fluorescein tracer test in the aquifer plus laboratory column data suggested that the very-aggressive flow conditions necessary to achieve 2.5 m of nZVI transport could be obtained using a hydraulically constrained flow path between injection and extraction wells. However, total unoxidized nZVI was transported only about 1 m and <2% of the injected nZVI concentration reached that distance. The experimental data also indicated that groundwater flow changed during injection, likely due to hydrogen bubble formation, which diverted the nZVI away from the targeted flow path. The leading edge of the iron plume became fully oxidized during transport. However, within the plume, oxidation of nZVI decreased in a fashion consistent with progressive depletion of aquifer "reductant demand". To directly quantify the extent of nZVI transport, a spectrophotometric method was developed, and the results indicated that deployment of unoxidized nZVI for groundwater remediation will likely be difficult.

Mechanism of gas bubble shoot-off and motion during spark discharge in liquid

The conditions of the excitation of a pulsed plasma discharge on the surface of a processed metal (copper) sample immersed in a conducting aqueous solution have been studied. Cathode polarization of the metal was provided by a high-voltage capacitor bank (4μF) charged to U = 200–1100 V after each discharge. It is established that electric breakdown with a duration not exceeding 0.1 μs is always preceded by the formation of small hydrogen bubbles (with radii r ≈ 37–40 μm) on the polarized metal surface, which takes about 139–159 μs. A mechanism of passivation of the processed metal surface by these hydrogen bubbles, which are synchronously shot off from the electrode surface under the action of the spark discharge, is proposed. Consistent matching of the experimental data and model calculations is used to estimate the main parameters determining the kinetics of bubble detachment from the electrode surface at various voltages U on the capacitor bank.

Removal of Natural Organic Matter from Wastewater by Electrocoagulation Using Aluminum Electrodes

Electrocoagulation is a promising alternative to the conventional chemical coagulation in water treatment systems. In electrocoagulation, coagulants are generated in situ by anodic dissolution of sacrificial electrodes, usually aluminum or iron electrodes. Anodic dissolution of the sacrificial anodes leads to the formation of hydrolysis products (hydroxo-metal species) that involve the destabilization of suspended, emulsified or dissolved pollutants and/or the formation of insoluble particles that adsorb and enmesh the pollutants. Furthermore, the formation of hydrogen bubbles as a result of water reduction at the cathode surface promotes the flocculation process by the soft turbulence in the system and produces a soft mix. The electrogenerated gaseous bubbles help the destabilized particles to colloid and generate larger particles which facilitate separation of the flocculated pollutants by carrying the particles to the top of the solution where they can be more easily removed by electroflotation. Electrochemical coagulation was successfully applied for turbidity, heavy metals, dyes and phenols removals from synthetic and real wastewaters and for breaking oil/water emulsions at both laboratory scale and pilot plant scale. In this work, effects of some experimental parameters (supporting electrolyte, current density, and initial pH) on anodic dissolution of aluminum and on electrocoagulation of tannic acid aqueous solutions as well as real industrial wastewaters containing tannic acid were investigated. Experimental results indicated that both chemical and electrochemical dissolution play an important role in the formation of hydroxo-aluminum species. The chemical dissolution of aluminum is strongly influenced by the solution pH. Electrocoagulation using aluminum electrodes achieved high removal efficiency of chemical oxygen demand from aqueous solutions containing tannic acid. The primary mechanism for removing tannic acid from water by electrocoagulation using Al electrodes involves the adsorption of tannic acid molecules on the aluminum hydroxide surface. Also, results of the treatment of real wastewater obtained from pulp and paper industry with initial COD concentration of 1450 mg/L have shown that more than 60 % of COD can be removed by electrocoagulation using Al electrodes under optimized experimental conditions. The specific energy required for the electrochemical process with Al electrodes was estimated to range from 1 to 2 kWh m-3.

Electrochemical Characterization of Thin Film Electrodes Toward Developing a DNA Transistor

The DNA-Transistor is a device designed to control the translocation of single-stranded DNA through a solid-state nanopore. Functionality of the device is enabled by three electrodes exposed to the DNA-containing electrolyte solution within the pore and the application of a dynamic electrostatic potential well between the electrodes to temporarily trap a DNA molecule. Optimizing the surface chemistry and electrochemical behavior of the device is a necessary (but by no means sufficient) step toward the development of a functional device. In particular, effects to be eliminated are (i) electrochemically induced surface alteration through corrosion or reduction of the electrode surface and (ii) formation of hydrogen or oxygen bubbles inside the pore through water decomposition. Even though our motivation is to solve problems encountered in DNA transistor technology, in this paper we report on generic surface chemistry results. We investigated a variety of electrode-electrolyte-solvent systems with respect to their capability of suppressing water decomposition and maintaining surface integrity. We employed cyclic voltammetry and long-term amperometry as electrochemical test schemes, X-ray photoelectron spectroscopy, atomic force microscopy, and scanning, as well as transmission electron microscopy as analytical tools. Characterized electrode materials include thin films of Ru, Pt, nonstoichiometric TiN, and nonstoichiometric TiN carrying a custom-developed titanium oxide layer, as well as custom-oxidized nonstoichiometric TiN coated with a monolayer of hexadecylphosphonic acid (HDPA). We used distilled water as well as aqueous solutions of poly(ethylene glycol) (PEG-300) and glycerol as solvents. One millimolar KCl was employed as electrolyte in all solutions. Our results show that the HDPA-coated custom-developed titanium oxide layer effectively passivates the underlying TiN layer, eliminating any surface alterations through corrosion or reduction within a voltage window from -2 V to +2 V. Furthermore, we demonstrated that, by coating the custom-oxidized TiN samples with HDPA and increasing the concentration of PEG-300 or glycerol in aqueous 1 mM KCl solutions, water decomposition was suppressed within the same voltage window. Water dissociation was not detected when combining custom-oxidized HDPA-coated TiN electrodes with an aqueous 1 mM KCl-glycerol solution at a glycerol concentration of at least 90%. These results are applicable to any system that requires nanoelectrodes placed in aqueous solution at voltages that can activate electrochemical processes.

EELS characterization of radiolytic products in frozen samples

Electron energy loss spectroscopy (EELS) was used to obtain information about the radiation chemistry of frozen aqueous specimens in the electron microscope by observing the hydrogen and oxygen K-edges. Measurements on frozen solutions of 30% hydrogen peroxide revealed the presence of molecular oxygen identified by a distinct 531-eV peak at the O K-edge even for electron doses below 100e/nm2. The molecular oxygen content of irradiated H2O2 solution was determined by least squares fitting of O K-edge reference spectra from water and gas-phase oxygen. It was found that the fraction of molecular oxygen to water oxygen was in the range 0.03–0.05. EELS from pure frozen water showed no features attributable to molecular oxygen or molecular hydrogen (K edge at ∼13eV) even at high electron doses above 105e/nm2. Spectra from frozen sucrose and protein solutions and their mixtures, however, did show evolution of a molecular hydrogen peak at ∼13eV for doses above 105e/nm2, consistent with previous measurements and indicative of hydrogen bubble formation. Molecular oxygen was not observed in any of the frozen solutions of organic compounds indicating that oxygen is not a major product of free radical decay, in contrast to molecular hydrogen formation.

Influence of Hydrogen on the Structural Stability of Annealed Ultrathin Si/Ge Amorphous Layers

The influence of hydrogen on the structural stability of multilayers made of ultrathin (3 nm) Si and Ge amorphous layers submitted to annealing to activate Si and Ge intermixing has been studied by TEM and AFM. By energy dispersive microanalysis the interdiffusion of Si and Ge has been observed. The Si/Ge multilayers, however, underwent remarkable structural degradation because of the formation of hydrogen bubbles which give rise to surface bumps and eventually craters when the bubbles blow up because of too high internal pressure in samples with high H content and annealed at high temperatures. The hydrogen forming the bubbles comes from the rupture of the Si-H and Ge-H bonds activated by the thermal energy of the annealing and by the energy released by the recombination of thermally generated electron hole pairs.

Vacancy trapping mechanism for hydrogen bubble formation in metal

We reveal the microscopic vacancy trapping mechanism for H bubble formation in W based on first-principles calculations of the energetics of H-vacancy interaction and the kinetics of H segregation. Vacancy provides an isosurface of optimal charge density that induces collective H binding on its internal surface, a prerequisite for the formation of ${\text{H}}_{2}$ molecule and nucleation of H bubble inside the vacancy. The critical H density on the vacancy surface before the ${\text{H}}_{2}$ formation is found to be ${10}^{19}--{10}^{20}\text{ }\text{H}$ atoms per ${\text{m}}^{2}$. We believe that such mechanism is generally applicable for H bubble formation in metals and metal alloys.